Waveguide to strip transmission line directional coupler



March 21, 1961 P. J. SFERRAZZA WAVEGUIDE TO STRIP TRANSMISSION LINE DIRECTIONAL COUPLER Filed May 14, 1958 2 Sheets-Sheet l 'IIIIIIIII/III VIIIIIIIIIII INVENTOR PET R J. SF RRAZZA /fi/ ATTORNEY March 21,1961 P. J. SFERRAZZA 2,976,499

WAVEGUIDE TO STRIP TRANSMISSION LINE DIRECTIONAL COUPLER Filed May 14, 1958 2 Sheets-Sheet 2 APERTURE ARRAY FOR NARROW ALL COUPLING CENIFRAL LONGITUDINAL APERTURE ARRAY FOR BROAD WALL COUPLING LONGITUDINAL INVENTOR AXIS ATTORNEY Tlzqfl B14 Mg PETER J. FERRAZZA:

WAVEGUIDE T STRIP TRANSMISSION LINE DIRECTIONAL COUPLER Peter J. Sferrazza, Wantagh, N.Y., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed May 14, 1958, Ser. No. 735,269

3 Claims. (Cl. 333-10) This invention relates to electrical wave transmission systems and more particularly to improved electromagnetic wave energy couplers providing a directional coupling characteristic between hollow waveguides and strip transmission lines.

Strip transmission lines consist generally of a thin conductive tape or a strip supported on a strip of dielectric material on the surface of a somewhat wider conductor which acts as a ground plane. In some cases, the line is provided with two ground planes, each facing one surface of a centrally located conductive strip sandwiched between two dielectric strips. Although strip transmission lines have the advantages of small size and light weight as compared to conventional hollow pipe waveguides, their power handling capabilities are limited. Thus, strip transmission lines may be used in the parts of a microwave system where the power level is low, and hollow pipe wave guides may be used in other parts of the same system where the power level is high.

It is the general object of this invention to provide an improved directional coupler for use between hollow waveguides and strip transmission lines.

Another object of this invention is to provide a hollow waveguide to strip transmission line directional coupler having a coupling coefiicient and good directivity which is not detrimentally affected by changes in frequency over a broad frequency band of energy transmitted in the hollow waveguide.

A still further object of this invention is to provide a waveguide to strip transmission line coupler having the ability to handle high main line power.

Another object of the invention is to provide a hollow waveguide to symmetrical or unsymmetrical strip transmission line coupler that is simple and inexpensive to manufacture.

These and other objects of the invention will become apparent as the description proceeds. In one embodiment of the invention, these objects are achieved by the provision of a waveguide transition for coupling from a rectangular waveguide operating in the TE mode to a strip transmission line operating in the TEM mode. Suitable directional coupling means such as a plurality of rectangular slots are disposed in one wall of the rectangular waveguide at a proper angle to interrupt the magnetic field.

Flat coupling and high power handling capacity may be obtained by arranging the coupling slots in a side wall rather than a broad wall of the rectangular waveguide. High directivities over a wide bandwidth are achieved with multiple coupling apertures having different magnitudes of coupling coefiicients in accordance with a suitable array, e.g., the coefiicients obtained from the binomial theorem or the Tschebychev expansion.

Directional couplers embodying this invention have been tested at 7 megawatts peak power in an L band waveguide and showed no evidence of breakdown. These couplers lend themselves to printed circuit techniques in the strip transmission line portion. Since they can be 2,976,499 Patented Mar. 21, 1961 built to use a portion of the outer surface of the waveguide as the ground plane for the strip transmission line,

these couplers add little to the size and weight of the vention;

Fig. 4 is a perspective view of a section of another type of strip transmission line adapted for use in the present invention.

Fig. 5 illustrates a strip transmission line termination; Fig. 6 is an expanded view partially cut away of another embodiment of the invention and is explanatory of the operation of the invention;

Fig. 7 is a perspective View of a third type of strip transmission line adapted for use in the embodiment of the invention shown in Fig. 6;

Fig. 8 is a diagrammatic view showing four apertures whose respective coupling magnitudes vary according to the coeificients of the binomial theorem, e.g., 1:3:321; and

Fig. 9 is a diagrammatic view showing an example of apertures placed side by side in pairs.

Referring now particularly to Fig. 1, a hollow rectangular waveguide 2 is provided with an elongated narrow opening 18, disposed in one of the waveguide narrow walls 5. Window 18 extends along a portion of the central longitudinal axis of wall 5. Strip transmission line 1 is mounted on the side of waveguide 2 covering said window 18. The strip transmission line 1, as shown in Figures 2 and 3, comprises two dielectric layers 10 and 12 with a thin strip of copper 11, or other suitable conductor sandwiched in between. The dielectric layers 10 and 12 are covered on their outer surfaces 9 and 13 with a thin layer of copper or other suitable electrical conductor. The conductor 9 which is contiguous to wall 5, Fig. 1, of hollow waveguide 2 contains apertures 15 serve to complete the circuit at the terminating end of strip transmission line 1. A connecting device 17 for providing a transition from the strip transmission.

line to a coaxial line is connected to the other end. Elongated narrow apertures 6, 7, and 8 are provided in the conductor 9 of strip transmission line 1 along that portion of the central longitudinal axis which communicates with hollow waveguide 2 through said window 18. The longitudinal axes of narrow elongated apertures 6, 7 and 8 are obliquely inclined to the longitudinal axis of rectangular waveguide 2.

Fig. 2 illustrates the juxtaposition of the two microwave guides.

Although Fig. 1 depicts as one embodiment of the invention a rectangular waveguide 2 coupled to strip transmission line 1, it is understood that all types of hollow waveguides may be similarly coupled to strip transmission line. In such cases, however, adjustments must be made with respect to dimensions, geometry, number and location of the coupling slots.

The strip transmission line shown in Fig. 4 may be used in the same manner as the strip transmission line shown in Fig. 3 in combination with hollow waveguide 2 to make a directional coupler. Fig. 4 depicts an unsymmetrical strip transmission line.

It is essentially the with the elimination of dielectric 12 and outer conductor Fig. illustrates in greater detail termination 22 of Fig. 1. Any resultant backward energy directed along conducting strip 11, Fig. 1, that arrives at termination 22 will be absorbed. The termination 22 comprises a narrow band of evaporated metal film 21, having a resistivity matching the impedance of the strip transmission line, centrally located on a heat resistant electrical insulator, e.g. a ceramic or glass wafer. Two bands of solder film 23 and 24 are placed on each side of the metal film. A thin protective coasting such as quartz may be provided to seal the metal film from the atmosphere. The strip transmission line center conductor 11 is connected to one soldered end 23 or 24 of termination 22. The other end of the termination is clamped between metal plates 14 and 15, Fig. 1. Plate 14 and ground plane 9 are bolted to metal waveguide wall 5, Fig. 1.

In the embodiment of the invention shown in Fig. 6, the coupling apertures are cut only into the walls of the hollow Waveguide. Note that this is the complement of the coupler shown in Fig. 1 where the apertures are out only into the strip transmission line. A strip transmission line 19 without ground plane 9 of Fig. 3 is mounted over the slots. The wall 50 of hollow waveguide 20 being contiguous to the dielectric wall of strip transmission line 19 serves as the second ground plane for the strip transmission line.

The strip transmission line depicted in Fig. 7 may be used with the hollow waveguide 20 to make a directional coupler in the same manner as described above with reference to Fig. 6. The strip transmission line shown in Fig. 7 is similar to that shown in Fig. 4 but without the ground plane conductor 9.

Although waveguide to strip transmission line couplers of the type shown in Fig. 1 having windows in the walls of the hollow waveguide are easier to manufacture than the type shown in Fig. 6 having slots in the walls of the hollow waveguide, the latter is more satisfactory for high power application.

Fig. 6 is also explanatory of the operation of the invention. The flow of current in the strip transmission line 19 is parallel to its longitudinal axis while the flow of current in the sidewall of the rectangular waveguide 26 is perpendicular to its longitudinal axis. Thus, the primary and secondary current flows are at right angles to each other. Calculations have shown that slots placed at a 45 angle to such perpendicular current flows give maximum coupling while slots placed in a position parallel to either current flow will give zero coupling.

Magnetic field 30, in the TE mode, is shown traveling forward in the main waveguide 20 in the direction of arrow 25. At slot 6 this energy is separated into two portions. One portion continues forward into the main waveguide 20. The other portion proceeds through coupling elements 6 into the auxiliary section of strip transmission line 19. This is due to slot 6 being placed at 45 to the magnetic field and causing thereby a perturbation in the magnetic field with associated fringing. The fringed mag netic field encircles the center conductor 11 of the strip transmission line 19 causing a current to flow in said center conductor where it divides into a forward wave and a backward wave. The forward wave 26 proceeds in the same direction of propagation of energy as in the main waveguide. The backward wave proceeds in an opposite direction towards an energy absorbing termination. As an alternative, a connecting means, such as the one shown as 17, Fig. 1, may also be provided for connecting the backward directed wave to an external coaxial cable.

The longitudinal axes of the slots shown in Fig. 6 make angles of 45 with the transverse plane to the waveguide. This arrangement will best achieve maximum center narrow wall coupling.

Fig. 8 depicts an arrangement of four apertures in a binomial array. The dimensions shown are suitable for an L band coupler. The ratio of the coupling coefficients of the apertures may be expressed as the coefficient of the binomial expansion, e.g., 1:2:1 for a three element array 1:3:3:1 for a four element array, etc. Fig. 8 illustrates an aperture array for maximum narrow wall coupling. Note that the longitudinal axes of the apertures make an angle of with the longitudinal axis of the array. The array is centrally disposed in the side of the waveguide.

The velocities of propagation in the primary and secondary lines of the directional coupler are unequal. Therefore the apertures are not spaced at quarter wavelength intervals as is usual in directional couplers, but are spaced so as to provide a round trip phase shift of 180 at the design frequency. This will cause cancellation of the backward waves. The forward waves will not combine exactly in phase, but this will not substantially affect the directivity of the coupler, and will cause only a slight increase in the attenuation. For example, the forward current from slot 6, Fig. 1, will add to the forward current from slot '7 along the strip transmission line because they are approximately in phase. The backward current from slot 6 will cancel the backward current from slot 7 because the distance between slots is such that the backward current from slot 7 will have traveled a distance equal to 180 and the two currents are then out of phase.

Consider the spacing between any two slots in a three element binomial array. Because of the difference in phase velocities of the two dissimilar transmission lines we may consider the spacing in electrical degrees between two slots in the strip transmission line to be 0 and 0., in the waveguide. Referring the phase of the waves to a reference established at the center of the binomial array, the vector sum of the forward coupled amplitude may be shown to be 2+2 cos (fl -9 (1) It may also be shown that the following is an expression for X, the spacing between slots for a total round trip of 180 between successive slots.

where x, and A are the respective wavelengths in the strip transmission line and waveguide.

Assume that it is desired to design the apertures for coupling a strip transmission line to a rectangular waveguide. The apertures are specified to be rectangular narrow slots placed along the central longitudinal axis of the narrow wall of the rectangular waveguide at an angle of 45 with the transverse plane.

Knowing the desired amplitude A of the forward coupled wave, it may be shown that the length and width of a single slot may be determined from the following expression:

2ax 2ab C15 where E is the relative dielectric constant of the strip line material;

w is the slot with;

l is the slot length;

D=w+ ln2;

C=thickness of strip transmission line;

a=width of hollow waveguide;

b=height of hollow waveguide; x =the guide wavelength in the rectangular waveguide; A=free space wavelength.

It can be shown that Formula 3 was derived from Bethes general expressions for coupling formulas found in MIT Radiation Lab. Report 194, March 24, 1943. Similar expressions may be derived for other applications involving such variations of coupling elements as rectangular slots in the broadwall, round irises in the broadwall, apertures in circular waveguides, aperture angles other than 45, etc.

From Equation 2, supra, the proper slot spacing X is determined. To obtain directivity, a three element binomial array (1 :2:1) may be selected.

From Equation 1, supra, the vector sum of the amplitude of the forward coupled waves normalized to the end element is:

2+2 cos (O where 0 and B are the slot spacings in electrical degrees. For a coupler of N db it may be shown that the coupling of the center slot is:

2+2 cos (0,,-

The coupling of the end slots is:

end enter db In a similar manner, the coupling values for an array of any arbitrary number of elements can be determined.

In the practice of the invention with rectangular hollow waveguides, it is preferred at present to couple through the narrow wall rather than the broad wall. However, broad wall coupling may be used. One suitable array for this purpose, shown in Fig. 9, consists of pairs of slots spaced along the center of the broad wall and extending transversely of the longitudinal axis. The dimensions given in Fig. 9 are suitable for an L band coupler.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

l. A microwave directional coupler comprising a section of rectangular waveguide transmission line having broad and narrow walls and adapted to propagate traveling waves of microwave energy in a transverse electric mode, a section of strip transmission line adapted to propagate traveling waves of microwave energy in the transverse electromagnetic mode, said strip transmission line having a strip conductor and a ground plane conductor, a narrow wall of said rectangular waveguide and said ground plane conductor being contiguous over portions of their respective lengths, the contiguous portions of said transmission lines having common successively spaced narrow elongated coupling apertures disposed therein for providing magnetic field coupling between said two transmission lines, said coupling apertures extending in a direction parallel to the direction of propagation of microwave energy in said rectangular waveguide and each of said coupling apertures having its longitudinal axis inclined at an oblique angle to the longitudinal axis of said rectangular waveguide, the round trip spacing between two of said spaced coupling apertures being substantially equal to electrical degrees at the design frequency to provide cancellation of backward coupled waves, said round trip spacing including one traversal between apertures in each of said transmission lines.

2. A directional coupler comprising a section of hollow rectangular waveguide transmission line having broad and narrow walls for propagating traveling waves of microwave energy in a transverse electric mode, a section of strip transmission line comprised of a strip conductor and a ground plane conductor extending parallel to a narrow wall of said rectangular waveguide, said narrow wall and said ground plane conductor being contiguous over portions of their respective lengths, a plurality of narrow elongated coupling apertures longitudinally spaced in the contiguous portions of said transmission lines for coupling microwave energy between said lines, the longitudinal axes of said coupling apertures being at oblique angles to the longitudinal axis of said rectangular waveguide to couple transverse current flowing across the narrow wall of said rectangular waveguide to a longitudinal current on said strip transmission line, the round trip spacing between adjacent coupling apertures being substantially equal to 180 electrical degrees at the design frequency to provide cancellation of backward coupled waves thereby directionally coupling microwave energy between said transmission lines, said round trip spacing including one traversal between adjacent apertures in each of said transmission lines.

3. A microwave directional coupler comprising a section of hollow conductively bounded waveguide transmission line for propagating traveling waves of microwave energy in a transverse electric mode wherein the electric field of said waves is polarized in a first direction transverse to the longitudinal axis of said waveguide, a section of strip transmission line comprised of a strip conductor and a ground plane conductor positioned on said waveguide along a longitudinally extending region where current flow in said hollow waveguide is in a transverse direction parallel to said first direction, said ground plane conductor and said longitudinally extending region of said hollow waveguide being contiguous over portions of their respective lengths, a plurality of narrow elongated coupling apertures longitudinally spaced in the contiguous portions of said transmission lines, the longitudinal axes of said coupling apertures being at oblique angles to said first direction to couple said tansverse current from said hollow waveguide to a longitudinal current on said strip transmission line, the round trip spacing between adjacent coupling apertures being substantially equal to 180 electrical degrees at the design frequency to provide cancellation of backward coupled waves thereby directionally coupling microwave energy between said transmission lines, said round trip spacing including one traversal between adjacent apertures in each of said transmission lines.

References Cited in the file of this patent UNITED STATES PATENTS 2,654,842 Engelmann Oct. 6, 1953 2,704,802 Blass Mar. 22, 1955 2,901,709 Fitzmorris Aug. 25, 1959 

